1. Technical Field of the Invention
This invention relates generally to welding apparatus and methods, and more particularly to apparatus and methods for forming weld connections, and for initiating self propagating exothermic reactions, such as in the process of forming the weld connections.
2. Description of the Related Art
Exothermic welding has become recognized as a preferred way to form top quality high ampacity, low resistance electrical connections.
Exothermic welded connections are immune to thermal conditions which can cause mechanical and compression joints to become loose or corrode. They are recognized for their durability and longevity. The process fuses together the parts or conductors to provide a molecular bond, with a current carrying capacity equal to that of the conductor. Such connections are widely used in grounding systems enabling the system to operate as a continuous conductor with lower resistivity.
Examples of self propagating exothermic reactions for exothermic welding are found in the CADWELD(copyright) process and the Thermit(copyright) process. CADWELD(copyright) is a trademark of Erico International Corporation, Solon, Ohio, U.S.A., and Thermit(copyright) is a trademark of Th. Goldschmidt A G, Essex, Germany. Exothermic mixtures are basically a combination of a reductant metal and usually a transition metal oxide. An example is aluminum and copper oxide which upon ignition supplies enough heat to propagate and sustain a reaction within the mixture. It is usually the molten metal product or the heat of this reaction which is then used to produce a desired result. The CADWELD(copyright) process produces, for example, a mixture of molten copper and aluminum oxide or slag. The molten copper has a higher density than the slag and is usually directed by a mold to join or weld copper to copper or steel to steel. The aluminum oxide slag is removed from the weld connection and discarded. Another common mixture is iron oxide and aluminum. Where only the heat of the reaction is used, the heat may be used to fuze brazing material, for example.
The exothermic reaction produces a large amount of heat. The most common way to contain the reaction, and to produce the weld or joint, has been to contain the reaction in a split graphite mold. A prior art welding apparatus 10 utilizing such a split graphite mold 12 is shown in FIG. 1. Referring to FIG. 1, the mold 12 includes an upper mold body section 14, a lower mold body section 16, and a mold cover 20. The conductors or items to be joined, such as the bars 22 and 24, are thoroughly cleaned and then placed in the appropriate location to project into a weld chamber 26 defined by the body sections 14 and 16 of the mold 12. The upper mold body section 14 includes a crucible chamber 30 above the weld chamber 26, connected to the weld chamber 26 by a tap hole 32. The mold body sections 14 and 16 are then securely closed and locked usually with a toggle clamp, and a metal disk 34 is positioned in the crucible chamber 30 over a tap hole 32. An appropriate amount of exothermic material 36 is emptied into the crucible chamber 30 on top of the disk 34, and a traditional starting powder or material 40 is sprinkled over the top of the exothermic welding material 36. The starting powder 40 is essentially a much finer exothermic material. The mold cover 20 is then closed and the reaction initiated by igniting the starting powder 40 by the use of a flint ignitor.
The starting powder or material 40 sprinkled on top of the exothermic material 36 has a lower ignition temperature and is easily ignited by the flint gun while the flint gun cannot normally ignite the exothermic material 36 directly. When the exothermic material 36 is ignited, the molten metal phase separates from the slag and melts through the metal disk 34. The molten metal then is directed via the tap hole 32 to the weld chamber and the conductors 22 and 24 to be joined. Once the metal has solidified the mold body sections 14 and 16 are opened and the slag is separated from the weld connection. The mold 12 is cleaned and readied for reuse for the next connection.
As suggested by the above, exothermic mixtures of this type do not react spontaneously and need a method of initiating the reaction. This initiation method involves generating enough localized energy to enable the reaction to begin. One method of initiating ignition is that described above, use of a starting powder and an ignition source such as a flint igniter. However, because of the starting powder""s low ignition temperature and deficiencies in handling and shipping, much effort has been made to find a reliable and low cost alternative ignition system for the exothermic material. A number of electrical systems have been devised which range from simple spark gaps to bridge wires or foils, to much more esoteric devices such as rocket ignitors. Such efforts are seen, for example, in prior U.S. Pat. Nos. 4,881,677, 4,879,952, 4,885,452, 4,889,324 and 5,145,106. For a variety of reasons, but primarily because of power requirements, dependability, and cost, such devices have not succeeded in replacing the standard starting powder/flint gun form of initiating the self propagating exothermic reactions. Another electrical ignition system is the system disclosed in European Patent Publication No. 875 330, owned by the assignee of this application, which is incorporated herein by reference in its entirety.
In addition, there are other difficulties inherent with the welding apparatus and method described above. Aside from the difficulties in handling and shipping the starting powder 40, there may be problems in handling and shipping the bulk exothermic material 36 itself. Properly measuring the exothermic material 36 may be both time consuming and susceptible to error. Further, he graphite molds utilized in prior art apparatuses, such as the mold 12 utilized in the welding apparatus 10 shown in FIG. 1, may be costly because of the amount of graphite involved, and because of the amount of machining need to produce the passages shown in the mold 12 of FIG. 1. Finally, a process such as that described above produces undesirable residues on surfaces of the mold 12. The residues require periodic cleaning of the surfaces of the mold 12, a labor-intensive process. Even with periodic cleaning, the formation of the residues may reduce the operational life of the mold 12. Finally, the cleaning itself may cause damage to the mold, also leading to reduced operation life of the mold 12.
Accordingly, it will be appreciated that improved welding apparatus and methods would be desirable.
According to an aspect of the invention, a self-contained crucible assembly includes a container having side walls and a fusible bottom; a refractory material lining the side walls of the container; an exothermic weld material within the container; and an ignitor extending into the container for igniting the exothermic material.
According to another aspect of the invention, an ignitor for weld metal material includes first and second metal strips and an insulating material between the first and second metal strips. The first metal strip has a perforation therein operative to create a spark plasma emanating from the perforation when a voltage is applied to the first metal strip. The second strip is stiffer than the first strip.
To the accomplishment of the foregoing and related ends, the invention comprises the features hereinafter fully described and particularly pointed out in the claims. The following description and the annexed drawings set forth in detail certain illustrative embodiments of the invention. These embodiments are indicative, however, of but a few of the various ways in which the principles of the invention may be employed. Other objects, advantages and novel features of the invention will become apparent from the following detailed description of the invention when considered in conjunction with the drawings.